Recently, as a personal computer and the like have become widespread, a digital still camera, digital video camera, and the like (hereinafter, simply referred to as “digital camera”) that can easily capture image information to digital equipment have entered a period of proliferation targeting at individual users. It is anticipated that such digital cameras will be widely diffused hereafter as image information input equipment.
Further, since solid state image pickup devices such as a CCD (Charge Coupled Device), a CMOS (Complementary Metal-Oxide Semiconductor), and the like make progress on the path to miniaturization, the digital camera is also required to reduce its size. Accordingly, it is strongly requested to make an image pickup lens, which occupies a maximum space in the digital input equipment, compact. The easiest method to reduce the size of the image pickup lens, is to miniaturize the solid state image pickup device. But, to miniaturize the solid state image pickup device, a light receiving device must be miniaturized, which makes it more difficult to manufacture the solid state image pickup device. Furthermore, miniaturization of the image pickup lens inevitably requires sophisticated performance.
In contrast, when the size of the image pickup lens is reduced while maintaining the size of the solid state image pickup device unchanged, the position of an exit pupil inevitably gets nearer to an image surface. When the position of the exit pupil gets nearer to the image surface, since off-axial light beams emitted from the image pickup lens are obliquely incident on the image surface at a large angle, the light converging performance of a microlens (array) disposed on the front surface of the solid state image pickup device cannot be sufficiently exhibited, from which a problem arises in that the brightness of an image is extremely different between the central portion of the image and the peripheral portion thereof. When it is intended to dispose the exit pupil of the image pickup lens at a far position to solve the problem, the size of image pickup lens cannot help being increased in its entirety.
Further, it is also strongly requested to reduce the cost of the image pickup lens due to recent price lowering competition. To cope with the above requests, there is proposed an image pickup lens composed of three lenses (disclosed in, for example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2001-272598), Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2004-163849), and Patent Document 3 (Japanese Unexamined Patent Application Publication No. 11-52227).